1. There is a treiglad meddal (soft mutation) when a verb comes immediately after a noun or pronoun, in phrases like: Rhaid i mi ____ . It doesn't matter how far away from the rhaid this happens, so the 'mynd' will still change in a sentence like this: Rhaid i fi, John, y tiwtor, y gath a'r ci fynd i'r dosbarth.

2. There is no treiglad meddal after peidio, e.g. Rhaid i mi beidio mynd.

3. Here is the rhaid pattern written in full. Notice the 3rd person, where the 'i' changes:

Rhaid i mi fynd - I must go
Rhaid i ti fynd - You must go
Rhaid iddo fo fynd - He must go
Rhaid iddi hi fynd - She must go
Rhaid i ni fynd - We must go
Rhaid i chi fynd - You must go
Rhaid iddyn nhw fynd - They must go

4. The pattern with Ar ôl i... is identical. Note that this can be in any tense. The same pattern happens after words like cyn (before), erbyn (by the time that), wrth (as).

ar ôl i mi fynd - after I go / after I went
ar ôl i ti fynd - after you go / after you went
ar ôl iddo fo fynd - after he goes / after he went
ar ôl iddi hi fynd - after she goes / after she went
ar ôl i ni fynd - after we go / after we went
ar ôl i chi fynd - after you go / after you went
ar ôl iddyn nhw fynd - after they go / after they went